The GIF file format allows one color to be treated as
transparent. You can either specify a point using
TransparentPoint or a color using this property. Because the
export process may re-map the colors on your canvas you should not
specify a transparent color directly. Instead you should specify a
point that will be transparent and the export process will infer
the color after any color mapping has been done. The default is for
no transparent colors, which is what happens when both properties
are null. If both properties are not null, TransparentColor takes
precedence.